HIGHLY SENSITIVE WIDE-BAND SEISMOMETER USING A LASER INTERFEROMETER

A wideband (dc to 2 kHz) highly sensitive seismometer using a Michelson laser interferometer has been developed. The horizontal acceleration of the ground is sensed by a suspended mirror; its position is then detected as the fringe signal of the light in the interferometer. The minimum measurable displacement with this seismometer is 3 X 10(-11) m/square-root Hz at 1 Hz (limited by the thermal fluctuation of air and the frequency noise of the laser) and 2 X 10(-15) m/square-root Hz above 2 kHz (limited by the shot noise of the light). One application of this seismometer is to use it for very sensitive vibration measurements, such as in seismic noise investigations of possible sites for a large-scale laser-interferometric gravitational wave detector.